This invention relates to an engine starter and, more particularly, to an engine starter in which an output rotary shaft driven by an electric motor is axially slided so that a pinion gear disposed on a front end of the output rotary shaft is brought into engagement with an engine ring gear.
One example of a conventional coaxial engine starter for starting an internal combustion engine is disclosed in Japanese Laid-Open No. 63-90665. According to the conventional coaxial engine starter 1 shown in FIG. 1, a d.c. electric motor 2 has a hollow armature rotary shaft 3 and a plunger rod 4a of a solenoid switch 4 positioned at the rear end of the d.c. motor 2 is inserted into an inner passage 3a of the armature rotary shaft 3 so that the inserted front end of the plunger rod 4a abuts against the rear end of an output rotary shaft 5 coaxially disposed at the front end of the armature rotary shaft 3. The output rotary shaft 5 is axially slidably and rotatably supported by a bearing 8 disposed within the inner passage 3a of the armature rotary shaft 3, whereby the output rotary shaft 5 can be pushed forward when the push rod 4a of the solenoid switch 4 is actuated.
It is seen that a sun gear 10a is formed on the outer circumference of the front end of the armature rotary shaft 3 and a plurality of planetary gears 10b are in mesh with the sun gear 10a. These planetary gears 10b also mesh with an inner gear 10c formed in the inner circumferential surface of the front bracket 7 and are rotatably supported by shafts 10d secured on a clutch outer member 9a of an over-running clutch 9. The sun gear 10a, the planetary gears 10b, the inner gear 10c, the shafts 10d and the clutch outer member 9a together constitute a planetary speed reduction gear 10 which reduces the rotational speed of the armature rotary shaft 3.
On the output rotary shaft 5, an over-running clutch 9 is mounted, of which clutch inner member 9b is engaged with the output rotary shaft 5 by helical splines 5a, so that the output rotary shaft 5 is allowed to axially slide while being rotated by the clutch inner member 9b. A compression spring 9c is disposed between the clutch inner member 9b and the splined portion 5a of the output rotary shaft 5. On the front end of the output rotary shaft 5, a pinion 6 which engages and disengages relative to an engine ring gear (not shown) is mounted by straight splines 6a formed in the inner circumference of the pinion 6 and splines 6b formed on the outer circumference of the output rotary shaft 5. A stopper 12 is mounted on the output rotary shaft 5 by a stop ring 13. A compression spring 14 is disposed between a step of the pinion 6 and the output rotary shaft 5. When the output rotary shaft 5 is moved forward, the pinion 6 meshes the engine ring gear to rotate the engine.
In the coaxial engine starter of the above construction, when it is desired to increase the torque for starting the engine or to reduce the size of the electric motor 2 by increasing the speed reduction ratio between the pinion 6 and the engine ring gear, the number of teeth of the pinion 6 may be decreased. However, when the number of teeth of the pinion 6 is decreased, the dedendum circle of the pinion is decreased, so that the thickness of the base portion of the pinion 6 or the diameter of the output rotary shaft 5 at the portion on which the pinion 6 is mounted must be decreased. Therefore, the strength of the pinion 6 or the output shaft 5 will become insufficient. Also, the splines 6a and 6b, the spring 14 as well as the stopper 12 with the stop ring 13 are necessary to mount the pinion 6 on the front end of the output rotary shaft 5, whereby the number of parts is increased and the assembly time is elongated.
On the other hand, while the above problem of insufficient strength of the pinion 6 or the output rotary shaft 5 can be solved by integrally forming the pinion 6 on the output rotary shaft 5, such the output rotary shaft with the integral pinion cannot be assembled into the clutch inner member 9b of the over-running clutch 10 because the output rotary shaft 5 has the large-diameter splined portion 5a which engages with the helical splines 9c formed in the inner circumference of the clutch inner member 9b of the over-running clutch 9.